Radiative lifetimes of highly excited states in rubidium

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Radiative lifetimes of highly excited states in rubidium

F. Gounand, M. Hugon, P.R. Fournier

To cite this version:

F. Gounand, M. Hugon, P.R. Fournier. Radiative lifetimes of highly excited states in rubidium.

Journal de Physique, 1980, 41 (2), pp.119-121. �10.1051/jphys:01980004102011900�. �jpa-00209223�

119

Radiative lifetimes of highly ^{excited states in} rubidium

F. Gounand, ^M. Hugon ^{and P. R. Fournier}

Centre d’Etudes Nucléaires de Saclay, ^Service de _{Physique Atomique,} BP N° 2, 91190 Gif , sur-Yvette, ^France

(Reçu ^{le 26} septembre 1979, accepte ^{le 24 octobre} 1979)

Résumé.

Nous avons mesuré les durées de vie radiative d’états S et D très excités (9 ~ n ~ 18) du rubidium.

Les résultats obtenus ainsi que ^ceux déjà publics pour les états P ^et F sont en bon accord avec les valeurs théo-

riques ^{calculées par} la méthode de Bates-Damgaard.

Abstract.

Measurements have been made of the natural radiative lifetimes of highly excited rubidium

(9 ~ n ~ 18) ^{S and D states. These} results, ^{as well as those} already published concerning ^P and F states, agree well with the theoretical lifetimes calculated by ^the Bates-Damgaard ^method.

J. Physique ⁴¹ (1980) ^{119-121 1 FTVRIER 1980,}

Classification

Physics ^Abstracts

32.70

In the past ^few years, measurements of the radiative lifetimes of highly ^{excited states of alkali atoms have}

been reported [1-3]. These results would clearly ^{be of}

interest for physicists working in atomic physics, plasma physics, astrophysics.

Moreover they ^allow interesting comparisons ^with

theoretical calculations. We have already published

measurements of the radiative lifetimes for some

Rydberg ^{P and F states} in rubidium [4, 5]. ^{In order to}

obtain a consistent set of lifetime values for a given alkali atom, ^we present ^here measurements concerning

the nS and nD states of rubidium (9 n 18).

Comparison ^is given between the obtained experi-

mental values and the theoretical ones calculated by using ^{a modified} Bates-Damgaard ^Method [6, 7]

(here after refered as B.D.).

We use the classical method of time resolved laser induced fluorescence [4]. ^{Rubidium atoms contained}

in a cell are excited in a given high-lying ^{nS or nD}

state by stepwise pulsed excitation via the first reso-

nant 5P state. For convenience the 5p3,2 ^{state is}

chosen as the intermediate state for the excitation of the nS levels, the observation of the population decay being ^{made on the nS -+} 5P 1/2 ^{line. The nD states}

are produced by using light pulses ^whose wavelengths correspond either, ^{to the} 5PI/2 --+ nD3/2 transitions for n 13, ^{or to the} 5P3/2 -+ nD3/2,5/2 transitions for n > 13; ^the population decay is observed on

these same lines. Such a choice is determined by ^the

fact that we are also interested in the study ^{of some}

collisional properties ^{of the nD states} (results ^concern- ing the collisional properties ^of both the S and D levels will be published ^elsewhere [8]). ^{Thus our}

reported ^{lifetimes refer to the} nD3/2 ^{states for n 13}

and to the nD states, without allowance for fine structure, for n > 13. This is of no importance ^since

the lifetimes corresponding ^{to the two fine structure}

levels are expected ^{to be} very close, ^{for a} given n ^value.

A high-speed photon counting system ^{allows the} determination of the decay of the excited.state _popu- lation as a function of time for a given ^rubidium

pressure. The extrapolation ^{to zero rubidium} pressure

yields the natural radiative lifetime. The experimental set-up, ^{shown in} figure 1, ^{has been} already widely

Fig. ^1.

Experimental set-up.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01980004102011900

120

described and needs no further explanations [5].

All the experiments ^were performed ^{at T}

^{520 K.}

The working ^rubidium pressure lies in the 3 ^x 10 - 6

to 9 ^x 10-4 torr range. The results shown in table I have been corrected for pile-up and thermal _escape effects. Great care has been taken to avoid parasitic

effects due to superradiance ^which might ^be important

for Rydberg ^states [9]. ^{Other causes of error have} already ^been widely ^discussed [5, 101. ^{The main} uncertainty ^comes from the determination of the rubidium _pressure which was determined from both the temperature of the side arm containing ^the liquid

rubidium and from absorption measurements of the

resonance rubidium line [4, 5].

Table I.

Experimental Itfetimes ieXp for ^rubidium

excited states. The _Tth values are from [7]. ^{See text} for Teai values. All values in 10-9 s.

The B. D. method is the simplest ^for obtaining

theoretical determinations of the radiative lifetimes for Rydberg ^{states. We have} already reported ^such

extensive calculations in the case of the alkali atoms [7].

Comparison ^is given, for all the measured S, P, ^D and F levels, between theoretical _ttb and experimental

Texp ^{values in} figure 2. For the S, ^{P and D levels the} texp ^{values are observed to} be somewhat smaller than the _Tlh values. This discrepancy ^{can be} partially

removed by taking ^{into account} the interaction of the

black-body radiation with the highly ^excited

atoms [11]. ^{In order} to account for this effect we have

computed the lifetime values _teal’ which depend ^now

on the temperature ^{of the} experimental cell, using

the equation

where _Tb represents the effect of the black-body

radiation. The evaluation of _zb [11] requires ^the knowledge ^{of the} dipole matrix elements between the level under study and all the other radiatively

Fig. ^2.

Radiative lifetimes of the rubidium S-D-F (T

⁵²⁰ K) and P (T

⁴⁶⁰ K) ^levels texp : 0; tth : A; teal (see text) : ^{x .}

connected levels. Using ^{the B. D. method} [7] ^{for this}

evaluation leads _{to ’teal} values which are certainly

less reliable than the _Tth values because the Tb ^values

are mainly determined by radial matrix elements

connecting closely spaced Rydberg ^{levels. It is well-}

known that the B.D. method is not well suited for the calculation of such matrix elements [12] which,

on the other hand, contribute only ^{a few} percent ⁱⁿ the _Tlh values. Despite ^the resulting uncertainty ^{on the}

’teal values it seems more appropriate ^{for our} purpose to compare the T.XP values with the _’teal values rather than with the corresponding Tth ^ones. This is done in

figure 2 where the _’teal values are observed to exhibit close agreement ^{with our} experimental ^data (note

that the black-body correction is small for the F

levels).

For the first time a wide set of lifetime values

concerning hydrogenic (i.e. ^F states) ^{as well as non} hydrogenic (S, ^{P and D} states) Rydberg ^{levels is} presented ^{for a} given ^{alkali atom.} Besides the interest, already mentioned, ⁱⁿ having ^such data, ^the compa- rison shown on figure ² between the ’texp and ^’tcal values supports well the conclusion that the B.D.

method provides good ^estimates (within ^{about 20} %)

for the radiative lifetimes of the highly ^{excited rubi-}

dium atoms, ^once the black-body radiation effect is taken into account. The results already ^obtained by

other _groups [1, ^{2, 3,} 11 ] indicate that this conclusion

probably holds also for the other alkali atoms.

Acknowledgments.

The authors would like to

thank Dr. J. Berlande for helpful ^comments.

121

References

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